Here's what I found so far.
1) when all control and adjacent control channel signals are "poor" the radio selects the strongest signal.
2) When the current control channel is above "poor", the signal must transition two levels for the radio to change. for example if the control channel is at ...

I am looking for details on the algorithm used in Astro trunked smartnet or smartzone portables and mobiles that decides when to switch sites. I am also trying to find out if the RSSI is picked off of the AGC or if it is taken from some other place in the circuitry.